Page 82 - Zero Net Energy Case Study Homes-Volume 2

CASE STUDY NO. 9
SILVER STAR APARTMENTS
Domestic Hot Water – The Solar Thermal System
(Opposite, top) Interior view of Community Room.
(Opposite, bottom) Interior view typical one bedroom apartment.
(Photo on following overleaf, p.70-71) South facade of the Silver Star Apartments.
(Above) View of roof with solar PV arrays and, in foreground, the angled solar thermal pan- els. (Photo courtesy of FSY Architects)
There are twenty-six (26) panels making up the solar thermal system that essentially preheats the domestic hot water (DHW) before delivering it to four storage tanks with heat pumps attached. The temperature is boosted there by the heat pumps, ready for use throughout the project.
Because of the moderate climate and relatively low heating and cooling loads, the energy de- mand for water heating is often a dominant part of the total energy demand in these types of buildings—estimated at one-third of the total. The designers therefore paid particular attention to the efficiency of the system design for this load.
The project program prescribed that use of natural gas at the site was not an option. This meant that a natural gas supply infrastructure at the site and buildings did not have to be built. The design team chose a solar thermal system over one that was simply more solar PV panels combined with heat pump water heaters. Solar thermal systems are 80% efficient in converting solar energy to hot water whereas solar PV is only about 18% efficient converting solar energy to electrical energy. Therefore, it was determined that the solar thermal system makes more ef- ficient use of roof space and that it would use the renewable energy more effectively.
The solar thermal system provides enough energy to offset 80% of that required for all the DHW of the project. The heat pump water heaters, which are used to raise the water temperature to that required for use, therefore consume only 20% of the electric energy normally required for all the water heating. (See the pie chart on p. 73, where DHW represents only 5% of the modeled energy use for the entire complex.)
Construction
A contractor was a member of the design team for cost-estimating and constructability consult- ing. Costs were estimated for certain design alternatives, which brought cost-effectiveness crite- ria into the decision-making process at a detailed level. This produced a cost-efficient integrated design.
The project was a traditional design-bid-build process. In the end, the consulting contractor was the successful bidder and proceeded with the actual construction.
Renewable On-Site Energy Supply
ACOF engaged the solar PV system consultant at the very beginning of the design process to collaborate with the project team in reaching the balance between energy-efficiency measures and the design of the solar PV system to achieve the target of 105% offset of site energy use (slightly net positive). The challenge was principally to fit as many solar PV panels on the roof as possible, given the additional space requirements for fire-department access, rooftop mechani- cal equipment and the solar thermal system.
To have space for enough solar panels, the roof was extended over the area between two of the three buildings and exterior walkways. It was not deemed cost feasible to place the panels on canopies over parking or vertically on the building. However, some PV panels were placed vertically against the south-facing building parapets for a boost in output. The result was the placement of a near horizontal array (tilted at 5°, facing due south) that is rated at 107 kW (DC) and a vertical array at the parapets (tilted at 90°, facing due south) that is reated at 20 kW (DC).
A battery storage system was designed as part of the on-site renewable energy system to allow load shifting and some measure of resiliency to the operation of the buildings during power out-
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Zero Net Energy Case Study Homes: Volume 2